1. Field of the Invention
The present invention relates to an apparatus and method for measuring the distance to an object utilizing light reflected by the object and particularly to an apparatus and method therefor capable of accurate measurement of the distance to a desired reflecting object irrespective of weather conditions.
2. Description of Prior Art
Various kinds of optical measurement apparatus utilizing lasers, etc., have recently been incorporated in ranging systems such as are used in vehicles to prevent collisions with obstacles on the road and to automatically follow a vehicle at a safe distance.
The above-described measurement apparatus recognize the presence of reflecting objects such as obstacles and preceding vehicles and measure the distance thereto on the basis of propagation delay time between emission of a light pulse and recepted of the reflected light pulse. The operation of such an apparatus is disclosed in Japanese Patent Application Unexamined Open No. Sho 58-203,524.
In such distance measurement apparatus, dust and/or water drops may adhere to optical elements such as lenses or the wind-shield and so lower the accuracy of measurement.
The adverse effects of this kind of soiling are especially prevalent in cases where the measurement apparatus is mounted on a vehicle. It is necessary to disposed the optical measurement apparatus in such a way that the optical elements can be shielded from soiling. To solve the above-described problem, such an optical measurement apparatus has been proposed in Japanese Patent Application Unexamined Open No. Sho 58-80511.
The distance measurement apparatus disclosed in the above-identified Japanese Patent Application is constructed as follows.
As shown in FIG. 1, a light-emitting element 2 and light-receiving element 3 are disposed near a front windshield within a vehicle compartment to shield the optical elements from dust and precipitation. This arrangement in particular takes advantage of windshield wipers, which clear the outer surface of the front windshield of precipitation. The optical elements 2 and are positioned opposite areas of the front windshield covered by the sweep of the wiper mechanisms to ensure accurate measurement. In FIG. 1, numeral 4 denotes a distance calculation section for calculating a distance to a reflecting object based on a propagation delay of a round-trip light pulse and numeral 6 denotes a wiper driving section which drives the wiper mechanism to clear the windshield.
However, there is another problem in the above-described conventional distance measurement apparatus. Light transmission can also be disrupted by air-borne weather conditions such as rain, fog, snow. etc. That is as illustrated in FIG. 2, light from the light-emitting element 2 may be scattered by light reflective particles 7 suspended in the air under such weather conditions as rain, fog, or snow so that some of the light from the light-emitting element 2 is reflected by such particles 7. Consequently, the measured distance will refer to the particles 7 by which the light beam is reflected instead of the preceding vehicle 1.